Recording disks such as magnetic recording disks or optical recording disks usually cooperate with a spindle or chucking mechanism in a disk drive, which rotates the recording disk in close proximity to a transducer or head. The head may read and/or write data onto the recording disk. The data is usually arranged in concentric or spiral data tracks on the recording disk. In the case of magnetic recording, a magnetic read head is positioned over the desired data track to read specific data.
It is important be able to locate data tracks accurately, so that the maximum signal strength reaches the read head. There are several factors which influence the ability to position the head over a given data track. Factors related to the recording medium are:
Track Density--there is a tendency toward making recording media of smaller diameters and increasing the number of tracks per inch of recording medium diameter in order to increase recording density.
Removability--it is also desirable to make recording media disks which are removable. That is they are contained in envelopes (like floppy disks) or cartridges (like 3.5 inch microfloppy disks and certain removable Winchester type disks) which can be inserted in a disk drive and later removed and transported. Each time a recording disk is removed from and later reinserted into a disk drive, there is a chance that the position of the circular data tracks may be slightly changed (i.e. made off-center).
Centering Hub--the type of hub at the center of a recording disk which mates with the disk drive spindle also has an influence over the track location. The centering hub could be simple as a circular hole slightly larger in diameter than the disk drive motor spindle. U.S. Pat. No. 4,600,955, FIG. 3, shows a recording disk having a centering hub known as the "V" block hub. In a V block, the hub has a triangular shaped hole which engages the circular disk drive spindle. Two sides of the triangle are rigid, and the third side is a resilient member which holds the spindle against the two rigid sides. Both the circular hole and the V block are subject to changing positions as the sides of the hole wear. Also, if the recording medium with a V block hub is changed from one disk drive to another with a slightly different spindle size, the resilient member may shift the data tracks to be slightly off center in the second disk drive.
U.S. Pat. No. 4,459,628 teaches a hub design for a rigid disk which features symmetrically spaced resilient flexure elements around a central opening in the hub for frictionally gripping and centering a drive spindle. The centering hub depicted in FIG. 8 of that patent has an equilateral triangle shape with slots disposed parallel to the sides of the triangle, the triangle and slots defining three resilient flexure elements, which are shown to engage the drive spindle at 3 points (1 at the middle of each side of the triangle).
Another type of hub which has been tried utilizes a collet with a number of spring fingers (e.g., five) around its inside diameter and loaded toward the center of the collet.
If one has recorded a number of concentric data tracks on a recording medium, removes it from the disk drive in which is resides, and then inserts it onto a disk drive spindle, the ability of the recording disk to center itself on the spindle so that the spindle corresponds exactly to the center of the data tracks may be called centering repeatability. The actual inaccuracy or maximum distance between a data track which is somewhat off center and the head which is reading the track is called off track, and may be expressed in micro inches or micro meters.
The centering hub can amplify spindle tilt or TIR (total indicated run out). One object of this invention is to minimize this amplification effect which can dominate measured off track in removable disk recording media.
One type of recording medium with which this invention is particularly concerned is stretched surface recording disks (SSR) which generally comprise a rigid annular support and a thin polymeric film (recording medium film) having a recording layer, suitably attached to the periphery of the support. Preferably, the recording medium film is annular in shape. The rigid annular support preferably has raised annular ridges at or near its inside and outside diameters and a base portion between the raised annular ridges. Usually, there are recording medium films on both sides of the rigid annular support. The recording medium film is stretched taut, out of contact with the base portion of the support and is attached to the support member at or near the inner and outer annular ridges. One patent describing SSR is U.S. Pat. No. 4,623,570.
In that patent it is explained that the recording medium film of an SSR is held in uniform tension. This type of stress is produced by stretching film over a circular ring or hoop.
SSR are superior to floppy disks in that a pressure pad is not required, and the recording surface has substantially the same overall stability as the relatively more massive support to which it is attached. In addition, the SSR provides a flat recording surface which may be deformed slightly to conform to a transducer head. However, in order to take full advantage of the characteristics of SSR, it is desirable to include a hub which tends to minimize off track in removable SSR.